In a wireless communication system, a transmission signal is amplified by a PA before being transmitted via an antenna. Currently, reduced conduction angle AB PAs, (i.e., similar to class B), are used for constant envelope modulation schemes, and increased conduction angle AB PAs, (similar to class A), are used for non-constant envelope modulation schemes. In order to enhance transmitter efficiency and reduce battery consumption, class AB PAs with sliding bias, (i.e., current and voltage), are also used. The typical power added efficiency (PAE) in a conventional PA is roughly 35% at maximum output power.
Theoretically, switch-mode PAs are capable of achieving 100% efficiency. Practical implementations of a switch-mode PA have been shown to achieve better than 50%, (typically 60%-70%), PAE. However, the dynamic range of a low cost switch-mode PA suitable for user equipment (UE)-grade transmitters is typically less than 40 dB.
A typical transmitter in 3rd generation partnership project (3GPP) for both the time division duplex (TDD) and frequency division duplex (FDD) must be able to support at least 80 dB of output power control range. Therefore, a switch-mode power amplifier alone cannot be used as the final stage of a UE-grade transmitter.
Another important parameter that is related to a PA is impedance matching to effect maximum power transmission to an antenna. It is important to ensure that the output impedance of the PA is matched to the input impedance of the antenna. When a mismatch occurs, the transfer of power from the PA to the antenna is decreased, which results in an inefficiency of the transmitter and increased battery power consumption.
In the case of a class AB PA with sliding bias, the PA output is conjugately matched to the load at maximum rated output power. The load condition is changed based on operation conditions of the antenna. In the sliding bias class AB PA, the output impedance of the PA changes with the direct current (DC) bias. The DC bias, as well as the output load, need to be dynamically adjusted with changing output power levels to optimize the class AB PA output match and thus maximize the efficiency. The load should be dynamically adjusted to compensate for both the effects caused by the changing bias conditions of the PA as well as changing load conditions presented by the antenna.